An example of a conventional signal read out circuit included in a magnetic recording device is described in U.S. Pat. No. 4,081,756 (granted to Robert Price et al.). FIG. 1 shows the circuit construction of the above example. The operation of the example will be explained below, with reference to a timing chart shown in FIG. 2. Referring to FIGS. 1 and 2, a reproduced waveform 1 from a magnetic head is applied to a gate generator 8, which produces gate pulses from a reproduced waveform 1 by using a slice level 9, to obtain peak pulses corresponding to peak positions of the reproduced waveform. The peak pulses are applied to a detector 4, which produces read data synchronized with the output signal of a VFO (variable frequency oscillator) and sends the read data to an upper control device. In the above signal read out circuit, however, there arises a problem that when the reproduced waveform 1 has a waveform distortion which is indicated by a broken line in FIG. 2, on the basis of a defect in a recording medium or noise, an erroneous gate pulse is generated, and thus the read data contains an error. When the slice level is raised so as not to generate a gate pulse at a signal portion having the waveform distortion, it is impossible to generate a gate pulse at an undistorted signal portion having a low signal level and thus an error is produced in the read data. As mentioned above, according to the prior art, it is impossible to discriminate between a signal portion and noise which are equal in absolute value of amplitude level to each other.